The more new things we know, the more things there are to be excited by, the more things there are to search for analogies for new experiences. We live in an age when rumours, ideas, actual new facts, are all available at the tap of a keyboard – we are almost too glutted for things to thrill us any more. Donne lived in an age of new learning, and new facts, but they were delivered sedately; he never had a chance to be overwhelmed by them, he had time to absorb them and make use of them.

We're used to his equation of intellectual excitement with erotic sensation in his love poetry. "Oh my America, my New Found Land", he says to his mistress. We are less likely to make the link between that excitement and his religious side. Yet part of the point of Renaissance humanism was to stay God-intoxicated even at the height of one's fascination with creation. There were only the first signs of the rift that dominated the next few centuries – in one of his satirical pamphlets, Ignatius His Conclave, Donne shows some sign of seeing it coming and worries that Copernicus's ideas might be damnable. More usually for him, finding things out is an act of worship; and a way of finding new language in which to pray.

Since I am coming to that Holy room, Where, with Thy choir of saints for evermore, I shall be made Thy music; as I come I tune the instrument here at the door, And what I must do then, think here before;

Whilst my physicians by their love are grown Cosmographers, and I their map, who lie Flat on this bed, that by them may be shown That this is my south-west discovery, Per fretum febris, by these straits to die;

I joy, that in these straits I see my west; For, though those currents yield return to none, What shall my west hurt me? As west and east In all flat maps – and I am one – are one, So death doth touch the resurrection.

Is the Pacific sea my home? Or are The eastern riches? Is Jerusalem? Anyan, and Magellan, and Gibraltar? All straits, and none but straits, are ways to them Whether where Japhet dwelt, or Cham, or Shem.

We think that Paradise and Calvary, Christ's cross and Adam's tree, stood in one place; Look, Lord, and find both Adams met in me; As the first Adam's sweat surrounds my face, May the last Adam's blood my soul embrace.

So, in His purple wrapp'd, receive me, Lord; By these His thorns, give me His other crown; And as to others' souls I preach'd Thy word, Be this my text, my sermon to mine own, "Therefore that He may raise, the Lord throws down."

Donne has no problem at all with mixed metaphor. Though this is mostly a poem about sacred and profane geography, it throws in music and medicine and that particular sort of medieval theology through analogy in which everything is a symbol of something else, and metaphor shows Christian doctrine as embedded in everything. In a very real sense, for him, there are no mixed metaphors in religious poetry, because everything is a symbol of the same things, the same truths, the same relationship between the soul and God. The agonies of sickness are the tuning up of the soul as a musical instrument, the winding of gut. The dying man, pointed to by his doctors like the corpse in Rembrandt's Anatomy Lesson, is a map of sickness, a chart of the ways through which we may navigate our way to salvation.

In a stock trope of earlier poetry – almost shockingly medieval to find in a mind as modern as we think of Donne as being – the fall of Adam is linked to its contrary, the crucifixion and redemption.

One of the possible reactions to all of this – for some believers as well as for the rest of us – is to find the red meat of doctrine too shocking to contemplate, or too absurd. Another reaction is to acknowledge that things this extreme and in some senses bizarre are precisely what Christians have mostly believed – that either there was an actual historical fall for which God sacrificed himself to himself in an act of vicarious atonement, or that, somehow, this is true metaphorically at some sort of remove as a way of talking about the relationship between our human sense of unworthiness and our sense of the transcendent, and our negotiations between the two. And once you have acknowledged this as a fact of intellectual history, what then? Accepting that this structure is culturally important is not accepting that it is objectively true – it is a convention as arbitrary and useful as even-tempered tuning. With the difference that a convention, like the key of C major, does not come with a lot of baggage about how we are to live our lives. One of the hard truths about being a non-believer reading religious poetry is that, however seductive it is, we stand outside it; we can even feel, when Donne stretches metaphor to breaking point, that he is struggling to convince himself.